Contemporary electronic control modules often need to interface with mechanical switches. In automotive applications, the switch interface circuits applied in these electronic control modules have many difficult specifications to meet. For instance, when a vehicle is powered off via an ignition switch, certain electronic systems must still be operative. An example of this is a electronic control module that senses a door lock switch. Since the electronic control module must sense an unlocking of the door lock switch, the module needs to be powered while the vehicle is powered off. This requires that the electronic control module, and its switch interface circuitry associated with sensing the door lock switch, must operate at a very low current so as not to deplete the vehicle's battery while the vehicle is not in use. This low current requirement becomes even more relevant when the electronic control module must interface to many switches as is the case in automotive applications.
Furthermore, the switch interface circuitry must be able to withstand an overvoltage condition known as load dump which is associated with an improperly working battery charging system. While typically operating at a relatively low voltage of typically 12 volts, the electronic control module, and its switch interface circuit, must withstand a load dump voltage of over 60 volts without being destroyed.
Also, since electronic control modules are often assembled at an automobile assembly plant, and are capable of being exchanged in the field at a garage facility, the electronic control module and its switch interface circuit must withstand a reverse battery hookup condition common in mis-installation situations.
Additionally, the switch interface circuit portion of the electronic control module must operate while powered over a relatively wide operating voltage, typically ranging from below 5.5 volts and over 14 volts, while maintaining accurate switch sensing.
FIG. 1 shows a typical prior art switch interface circuit 100 in an automotive control module application. A door lock switch 101 is connected in a high-side configuration between a voltage source 103 and the switch interface circuit 100. The voltage source 103 is provided directly from a vehicle's battery. This voltage source 103 can range in amplitude from 5.5 volts when the vehicle's battery is either heavily loaded, such as it is during a cold crank condition, or substantially depleted, to more than 14 volts when the vehicle's alternator is charging the battery.
The door lock switch 101 is connected to the switch interface circuit 100 through an input terminal 105. The input terminal 105 is connected to a resistive divider comprising resistors 107 and 109. Resistor 107 is also connected to a clamp diode 115 that is connected to a voltage terminal 117 which also powers a comparator 113.
The comparator 113 outputs a voltage at a terminal 119 indicative of the physical state of the door lock switch 101. When the door lock switch 101 is in a closed position, the voltage source 103, through the resistors 107 and 109, cause the comparator 113 to output a switch closed state. Resistor 109 is also connected to a ground terminal 111, and is used to guarantee current flow to a ground state into the comparator 113 when the door lock switch 101 is in an open condition. This open condition causes the comparator 113 to output a switch open state.
The connected junction of the resistor 107, the resistor 109, and the clamp diode 115 is connected to an input of the comparator 113. The clamp diode 115 is in place to prevent an over-voltage condition, such as the earlier-mentioned load dump from destroying the comparator 113. The clamp diode 115 acts to limit any input voltage present at the input terminal 105 (which can exceed 60 volts as mentioned earlier) to a voltage present at the voltage terminal 117, which is typically 5 volts. The resistor 107 acts to limit current flow through the clamp diode 115 so as not to over burden a power supply regulator 121 driving the voltage terminal 117. Typically power supply regulators are not adept at sinking current as is the need when the current flows through the clamp diode 115. Additional regulator circuitry is often necessary when relatively low regulator load currents and relatively high clamp diode clamp currents coexist.
Note that the voltage source 103 has a significant amount of electrical noise emanating from it. In other words there is a significant amount of alternating current signal present on the underlying direct current voltage provided at the voltage source 103. Sources that contribute to the noise include transient loading of the vehicle's charging system that does occur when the vehicle is being started, when an air conditioning compressor turns on or off, and electromagnetic ignition generated noise, to name a few. This noise gets injected through resistor 107 and clamp diode 115 onto the voltage terminal 117. Since the voltage terminal 117 is directly connected to the comparator 113, if the noise exceeds the voltage output from the power supply regulator 121, the noise will be injected into an affect the operation of the comparator 113. Furthermore, the noise will be injected into the power supply regulator 121 and all other circuits connected to the power supply regulator 121. This can cause a lot of problems in electronic control modules because the power supply regulator 121 is usually connected to many circuits in the electronic control module.
Additional concerns with the prior art circuit shown in FIG. 1 include field reliability, compactness, and ease of manufacture. Ideally, to be reliable the number of discrete components should be reduced significantly. One approach to achieve these goals is to integrate the discrete components onto a silicon based circuit. Unfortunately, resistor 109 is typically made to have a relatively large resistance. Silicon integration of large resistances not only takes a significant amount of physical space, but the ability to repeatably fabricate a particular large resistance value is not practical in a silicon embodiment.
What is needed is an improved switch interface that has low current drain, is able to withstand an overvoltage condition, can withstand a reverse battery hookup condition, is not affected by relatively large amounts of noise injected, is reliable, compact, easy to manufacture, and accurately sense switch position.